This invention relates to a method and apparatus for analyzing positron extinction or annihilation which analyzes a sample by utilizing a positron extinction analytical method and to an electron microscope equipped with such an apparatus, and more particularly to technique which irradiates highly accurately positrons to an analytical position of a sample.
A positron extinction method is an analytical method which implants positrons into a sample, causes pair extinction or annihilation of the positrons with electrons in the sample and senses the .gamma.-rays emitted thereby in order to obtain various data such as an electron structure and lattice defect in the sample, and the like. The positron is a particle which has the same rest mass as that of the electron, whose absolute value is equal to that of the charge of the electron (negative charge) but whose polarity is opposite to that of the electron. A positron source uses a radioisotope (e.g. .sup.22 Na) which is sealed in the source and causes +.beta. decay. When the positron having energy of several hundreds of keV emitted from this positron source is radiated to a sample, the positron jumping into the sample impinges repeatedly against ion and electron, whereby the positron is converted to thermal energy of about .about.0.025 eV within a short period of about 10.sup.-12 seconds. When the sample is a metal, the positron thus converted to thermal energy causes pair extinction together with electron with life of from 100 to 300 PS.
At this time, two .gamma.-rays of about keV (that is, the energy corresponding to the rest mass of the positron and electron) are emitted in substantially the opposite directions. The angles and energy of the two .gamma.-rays depend upon the momentum that the positron-electron pair has at the time of the pair extinction (it is possible in this case to neglect substantially the momentum of the positron and to take into consideration only the momentum of the electron) from the law of conservation of the total energy and the total momentum including the mass at the time of pair extinction, and the two .gamma.-rays are distributed within an angular range (within .+-.25 m rad) with direction of 180.degree. being the center. Therefore, the energy, too, exhibits a Doppler width with 511 keV being the center. Accordingly, the momentum distribution of the electron in the sample can be determined by either measuring accurately the angular distribution of the two .gamma.-rays (measurement of .gamma.-.gamma. angular correlation) or measuring the energy distribution of the .gamma.-rays (Doppler width).
Hereinafter, a conventional positron extinction analytical method (refer to "Lattice Defect", Lectures On Experimental Physics (published by Kyoritsu Publication Co.), p. 163-175 (1978)) with reference to FIGS. 19 and 20 of the accompanying drawings.
FIG. 19 shows a conventional apparatus for measuring the energy spectrum of extinction .gamma.-rays (which are generated by the pair extinction of the positron and the electron). The positron 21 emitted from a positron source 12 (e.g. .sup.22 Na) is radiated to a sample 34, the extinction .gamma. rays 22 are detected by a .gamma.-ray detector 19 and the energy spectrum of the extinction .gamma.-rays is measured. The energy of two photons (.gamma.-rays) emitted at the time of extinction is both about 511 keV. The kinetic energy of the electron as the counter-part of the pair extinction is only a few eV, and for this reason, the energy of the extinction .gamma.-rays deviates from 511 keV. In a laboratory system, the energy width .delta..sub.E of the energy spectrum of the extinction .gamma.-rays is CP.sub.l /2 due to the Doppler effect. Here, C is a light velocity and P.sub.l is a momentum component in the emitting direction. Accordingly, the energy width .delta..sub.E expands to about 2 keV and can be measured by use of a Ge(Li) solid state detector (SSD) having high resolution, for example, as the .gamma.-ray detector 19.
FIG. 20 shows schematically the energy spectrum of the extinction .gamma.-rays. An S parameter is used for measuring the energy spectrum of the extinction .gamma.-rays. This S parameter is expressed by the following equation using the area A at the center of the spectrum and the areas B.sub.1 and B.sub.2 at both end portions in FIG. 20: EQU S=A/(B.sub.1 +B.sub.2) (1)
The center portion A of the spectrum is a region to which the .gamma.-rays resulting from the pair extinction of the positron and the free electron-like electron contribute primarily, while both end portions B.sub.1 and B.sub.2 are those regions to which the .gamma.-rays resulting from the pair extinction of the positron and the inner shell electron contribute primarily. The measurement of the S parameter is extremely effective means for studying the lattice defect, particularly the lattice defect having a low ion concentration such as a tensile stress portion of lattice vacancy, edge dislocation.
Let's consider the atomic void in a metal by way of example.
(1) Since the atomic void is formed by pulling out a cation, it is charged negative in comparison with a complete crystal. Since the position is charged positive, it is attracted to the atomic void due to the Coulomb mutual action when it comes near to the atomic void. When the atomic void concentration is about 10.sup.-14, almost all the positrons are attracted to the atomic voids and disappear near the atomic voids.
(2) The potential energy of the electron alone due to the fall-off of the cation at the atomic void is almost flat and the electron becomes like a free electron. Therefore, the greater the number of positrons that disappear at the atomic voids, the greater becomes the pair extinction with the free electron-like electrons and the greater becomes the S parameter. (The center portion A of the spectrum becomes relatively greater than both end portions B.sub.1 and B.sub.2).
(3) When a plurality of atomic voids gather and form a void cluster, the electron becomes further like free electron and the S parameter becomes further greater.
As described above, the S parameter is effective for examining the generation of the atomic voids and the recovery process. For instance, when electrons of 3 MeV are radiated at 10K to copper (Cu), the S parameter increases with the radiation. This is because the copper atom is sprung out from the lattic point by the high speed electron, thereby forming the atomic void. When the damage caused by radiation is recovered by raising the temperature after the radiation, the S parameter increases at a stage III (a temperature range in which the atomic void can move). (Refer to S. Mantle et al: "Phys. Rev. Left", 34, p. 1554 (1975).) This is because the atomic voids move at the stage III and the void cluster is formed thereby.
Besides the extinction or annihilation .gamma.-ray energy spectrum measurement described above, the positron extinction or annihilation method includes .gamma.-.gamma. angular correlation measurement and positron life measurement. (For example, when the positron is trapped into the empty lattice point from which the lattice atom of a metal crystal lattice falls off, it is known that the life extends. Therefore, the empty lattice point concentration in the metal can be determined by measuring the life of the positron.) They are also extremely effective means for studying the lattice deffect in the same way as the method described above. However, since all of these conventional positron extinction methods simply irradiate the positrons emitted from the positron source to the sample, the resulting data are unavoidably averaged data for a wide range because the positron radiation surface of the sample is generally from several mm.sup.2 to several cm.sup.2.